The invention relates to a method for testing connections of metallic workpieces with plastic compounds for cavities using ultrasound.
Plastic compounds are to be understood in the following as relating to any type of plastics which may find application e.g. as adhesive for bonding metallic workpieces, as seal, or as coating materials for metallic workpieces. For example, these materials are used for realizing adhesive bonds when metal sheets or tubes are involved or coating thereof.
When workpieces, such as for example metal sheets or tubes, are coated with a plastic, the application may cause defects in the form of e.g. cavities (air bubbles) in the coating or as a non-adherent connection at the contact point between coating and workpiece. Such defects can cause filiform corrosion of the workpiece and thus lead to a premature breakdown as a result of pitting corrosion.
When producing adhesive bonds, e.g. when tubes are involved, it is known to connect the tube ends with one another by means of a sleeve through introduction of adhesive into a joint gap between sleeve and tube.
Typical flaws of such bonds are for example air bubbles and non-adherent connection zones (so-called kissing bonds).
Strength tests of bonded tube connections have shown that these flaws for this application become critical only when exceeding a size of about 20 mm. It has been further shown that even though air bubbles in particular of this size normally fill the entire height of the joint gap, they are enveloped by a thin adhesive skin (100 μm-200 μm thickness) which adheres to the tube wall.
In order to ensure a sufficiently firm and flawless connection of the tubes, adhesive bonds are typically inspected by means of ultrasound using normal sound incidence. The sound is coupled into the first joint partner (sleeve) and travels there through. At the interface to the adhesive layer, part of the sound is reflected whereas another part penetrates the adhesive. The part that penetrates causes a further echo when reflected at the backside of the adhesive layer. In contrast thereto, the part that is reflected at the top surface of the adhesive layer causes in contrast thereto a long echo train through repeated reflections in the sleeve wall.
In the simplest solution of ultrasonic inspection of adhesive bonds of tubes, as known from JP 57037257 A, amplitudes of the echo from the backside of the adhesive layer are used for analysis. Sound is hereby sent through the adhesive and the absence of the echo indicates a flaw.
In the evaluation of the reflected ultrasonic signals, the latter are converted into electric signals and selected, amplified, and evaluated via time windows and orifices. The evaluations relate to amplitudes of the signals and the running times between the signals.
The examined information, i.e. the amplitudes, limit value exceedance, running time, etc., is detected, analyzed, logged by an electronic data processor and displayed on a monitor.
Other solutions, known e.g. from JP 2000221173 A analyze echoes from the top surface of the adhesive layer. Evaluated are hereby in particular the shape of the echo signals to draw conclusions about the quality of the adhesive bond.
As the ultrasonic signals get substantially weaker also after the adhesive layer or the coating material has cured, all solutions based on through transmission have the drawback that the echoes from the bottom side of the adhesive layer or the coating material can become very small especially when greater layer thicknesses are involved.
Experiences have shown that echoes from the bottom side of the layer can no longer be detected when layer thicknesses of above few millimeters are involved. In addition, in the case of adhesive sleeve bonds, the small back side echo superimposes the echo train from the sleeve which echo train produces in turn great amplitudes.
The methods based on the detection of the phase of the echo from the top surface of the adhesive layer suffer several shortcomings. On one hand, the reliable automated detection of the phase position is very difficult. On the other hand, there is no change in the phase position at the transition steel/plastic to steel/cavity especially for metallic workpieces as a result of the great acoustic density.
Further complicating the situation is the fact that formed cavities are always located inside the plastic compound and a thin layer of the plastic compound also always exists at the interface to the metallic workpiece in the form of a skin in surrounding relationship to the cavity so that the transition steel/cavity does not occur. The ultrasonic signals thus always impact the material combination steel/plastic/cavity in the presence of such flaws so that a meaningful ultrasonic inspection is complicated.
This is also a problem in the presence of air pockets in plastic coatings which may also be enveloped by a thin skin of plastic.
A change in the bonding or coating parameters (thickness of the adhesive layer or coating, condition of the workpiece surface, etc.) causes fluctuations of the ultrasonic signals which can easily lead to misinterpretations when undertaking only localized analysis.
In summary, ultrasonic inspection faces the following problem when detecting flaws in connections of plastics with metallic workpieces:    1. Air bubbles developing during coating of a workpiece or during filling of a joint gap with adhesive are encountered within the plastic compound and have therefore at the interface to the workpiece always a plastic skin which seals off the cavity.    2. Even in the presence of such a cavity, there exists therefore always the transition steel/plastic but not the transition steel/air (cavity). The detection of air bubbles beneath the plastic compound is therefore complicated especially for methods which use the reflection from the interface in the metallic workpiece.    3. Changes of the thickness and surface condition of the metallic workpiece as well as interference effects cause variations of the echo amplitudes. The localized analysis of these amplitudes and their relationships in the absence of accounting for proximal criteria becomes therefore unreliable.
It is therefore an object of the invention to provide a reliable and cost-efficient method for testing connections of metallic workpieces with plastic compounds by means of ultrasound to obviate the drawbacks of the known methods so that cavities in the plastic layer can be unambiguously detected and evaluated.